Crude oil of naturally low sulfur content is known as sweet crude and has traditionally commanded a premium price. The removal of sulfur compounds from transportation fuels has been of considerable importance in the past and has become even more so today due to increasingly strict environmental regulations relating to the release of sulfur-containing combustion compounds into the atmosphere.
Sulfur in fossil fuels is highly undesirable because of its potential to cause pollution, i.e., SOX gases and acid rain. Sulfur also results in the corrosion of metals and the poisoning of the precious metal catalysts that are widely used in the petrochemical industries. The United States Environmental Protection Agency has recommended strict regulations for the sulfur content in the diesel fuel used in the United States. According to these recommendations, the sulfur content in diesel fuel must be reduced from the current level of 500 ppm to 15 ppm during 2006. New regulations in Japan and in Europe require the reduction of sulfur in diesel transportation fuel to 10 ppm during 2007 and 2009, respectively.
Conventional hydrodesulfurization (HDS) processes have been used widely in refineries to transform sulfur-containing compounds mainly to hydrogen sulfide which itself presents a significant health hazard and is corrosive, particularly in the presence of water. When contacted with certain functional catalysts, hydrogen sulfide and other sulfur compounds act as a catalyst poison, that is, the sulfur deactivates or reduces the effectiveness of the catalyst. The breakthrough of sulfur from various sweetening processes results in catalyst poisoning, corrosion of tanks, ships, and pipelines, and can result in economic losses to the refinery from flaring, reinjection for reprocessing, or discounted sales prices for off-spec hydrocarbon products having high sulfur content.
The hydrodesulfurization process involves high temperature, elevated pressure, metal catalysts and large reactors. Apart from being an energy-intensive process, HDS has some inherent problems in the treatment of aromatic hydrocarbon sulfur compounds, such as dibenzothiopene (DBT), and their methylated derivatives, such as 4-methyldibenzothiopene and 4,6-dimethyldibenzothiopene (4,6-DMDBT). These compounds cause steric hindrance because their C—S bond energy is almost equal to the C—H bond energy, which makes them hard to break down by mere hydrotreatment.
An important factor for deep desulfurization is the reactivity of aromatic sulfur compounds. Deep HDS may produce low-sulfur diesel, but ultimately results in higher energy costs and the generation of CO2, which is a greenhouse gas.
HDS processing is not effective in completely removing the refractory sulfur compounds in diesel which are present in the form of n-alkyl benzothiophene and n-alkyl dibenzothiophene, where n is methyl, ethyl, or a mixture of both in different ratios and positions on the phenyl groups. The HDS process is not effective in the so-called deep de-sulfurization or deep removal to 10 ppm, or less by weight.
There are also references in the technical literature to processes for petroleum oil desulfurization. For example, Guth et al. disclose the use of nitrogen dioxides followed by extraction with methanol to remove both nitrogen and sulfur-containing compounds from petroleum feedstocks. (See Guth, E. D. et al., Petroleum oil desulfurization. 1975, (KVB Engineering, Inc., USA). Application: US. p. 8 pp.) Tam et al. describe a process for purifying hydrocarbon aqueous oils such as shale oils to remove heteroatoms impurities including nitrogen and sulfur compounds. (See Tam, P. S., Kittrell, J. R., Eldridge, S. W., Ind. Eng. Chem. Res. 1990, pp. 29, 321-324) Deshpande et al. disclose that ultrasonic methods can be applied for the intensive mixing of the biphasic system resulting in a reduction of more than 90% of dimethyl dibenzothiophene (DMDBT) contained in diesel fuel samples. (See Deshpande, A., Bassi A. and Prakash A., Ultrasound-Assisted, Base-Catalyzed Oxidation of 4,6-Dimethyldibenzothiophene in a Biphasic Diesel-Acetonitrile System. Energy & Fuels, 2005. 19(1): p. 28-34.
Yazu et al. have reported that dibenzothiophene can be oxidized effectively with hydrogen peroxide in the presence of 12-tungstophosphoric acid (TPA) in a n-octane/acetonitrile biphasic system to give their corresponding sulfones as the major product.
Liquid-liquid extraction is widely used to separate the constituents of a liquid solution by introducing another immiscible liquid. In the petroleum industry, solvent extraction has been used to remove sulfur and/or nitrogen compounds form light oil. The extracted oil and solvent are then separated by distillation. (See Yazu, K., M. Makino, and K. Ukegawa, Oxidative desulfurization of diesel oil with hydrogen peroxide in the presence of acid catalyst in diesel oil/acetic acid biphasic system. Chemistry Letters, 2004. 33(10): p. 1306-1307); Yazu, K., et al., Tungstophosphoric acid-catalyzed oxidative desulfurization of light oil with hydrogen peroxide in a light oil/acetic acid biphasic system. Chemistry Letters, 2003. 32(10): p. 920-921; Yazu, K., et al., Oxidation of Dibenzothiophenes in an Organic Biphasic System and Its Application to Oxidative Desulfurization of Light Oil. Energy & Fuels, 2001. 15(6): p. 1535-1536.
The processes of the prior art as reported in the literature are complex and present operational difficulties when practiced on an industrial scale. It has been shown that the oxidative desulfurization process using H2O2 or a related agent as the oxidant can be realized using either a heterogeneous or a homogeneous catalyst. A heterogeneous catalyst cannot contact the feedstock mixture of H2O2/H2O and the transportation fuel uniformly even in a fluidized bed reactor, since they exist in separate phases. Contact may catalyze the decomposition of H2O2 before it can react with the sulfur. The most commonly reported homogenous catalyst systems for efficiently promoting ODS are heteropolyanion catalysts. Heteropolyanion catalysts need a special medium to stabilize the catalyst and this type of catalyst is relatively expensive.
Despite the disclosure of numerous processes in the prior art, these processes have failed to provide low sulfur hydrocarbon fuels in an efficient and economical manner. Catalyst-based processes disclosed in the prior art employ catalysts that are complex, expensive to produce, and that are not recyclable. The use of these catalysts and processes for the mandated reduction in sulfur levels which are characterized as deep desulfurization, will be expensive to practice and will necessarily add to the cost of the transportation fuels. The use of complex, unstable and expensive catalyst compounds and systems that are non-regenerable and that can involve hazards in their disposal are less than desirable.
It is therefore an object of the present invention to provide a catalyst and process for deep desulfurization that produces essentially sulfur-free hydrocarbons with a chemically simple, inexpensive and reusable catalyst in a system that is highly efficient at low temperature and pressure.
It is another object of the invention to provide a process and catalysts that are efficient and economical for use on an industrial scale to achieve the deep desulfurization of such difficult to remove petroleum fuel components as the benzothiophenes and di-benzothiophenes.
It is a further object of the invention to provide a catalyst for use in the desulfurization process that is both robust and that can be readily regenerated and recycled for repeated subsequent uses in the desulfurization process.
Another object of the invention to provide an improved catalyst-based process that can be installed downstream of the HDS unit for the deep desulfurization of liquid distillate fuels.